Electric power systems typically comprise several power transformers as apparatus. Such power transformers can be either used as step-up or step-down transformers, which mean they are used to either increase or reduce the voltage level of an electrical circuit. Power transformers are generally used by electric utilities in generation, transmission and distribution applications as well as by large energy consumers such as, but not limited to, industries, oil and gas, railway operators, large buildings and facilities.
When a power transformer is de-energized, a residual magnetic flux may remain in the core of the power transformer. It is generally well known that due to that residual magnetic flux, the uncontrolled energization of a transformer may cause inrush currents having several orders of magnitudes of the rated current value of the transformer. Such event may increase or cause stress on electrical apparatus including, but not limited to, power transformers and associated circuit breakers. Induced stress may lead to premature wear of such equipment and may affect the reliability of the power system or plants and potentially lead to major blackout or plant downtime.
Over the years, techniques were developed to mitigate and/or reduce inrush current events. A well-known technique to mitigate power transformer inrush current uses a CB equipped with pre-insertion resistors/closing resistors. Another current technique for mitigating inrush current uses smoothing inductors along with the CB. However, these two known techniques require using more complex CBs with additional components and have proved to add major costs for installation and maintenance. Therefore, it is well-known that these mechanical add-ons increase the frequency of maintenance operations and reduce overall reliability.
A paper entitled “Elimination of Transformer Inrush Currents by Controlled Switching—Part I and II” published in the IEEE transactions on power delivery, Vol. 16, No. 2 in April 2001, discloses a new approach making use of controlled switching techniques. This paper describes a method for controlling the closing of a circuit breaker at a precise electrical angle calculated based on the magnitude and polarity of the residual magnetic flux of the transformer. Such paper also demonstrated that using CB with simultaneous pole operation may mitigate the inrush current at the closing moment of the CB in relation to the residual magnetic flux. Such method may be achieved by simultaneously closing the three phases of the CB at an optimum point as a function of the residual magnetic flux pattern. This technique requires that the residual magnetic flux in all three phases are known and that the residual magnetic flux magnitudes of two phases are higher than a certain threshold and follow the typical residual magnetic flux pattern (+r, −r, 0). Therefore, this technique limits the scope of use of such method.
Another paper, entitled “Transformer controlled switching taking into account the core residual flux a real case study” and published in CIGRE 13-201 session 2002, discloses demonstrated field results of the implementation of above mentioned technique. The controlled switching using independently-operated pole circuit breaker has proved to effectively eliminate the inrush current. This approach uses different closing electrical angle of the circuit breaker according to the calculated residual magnetic flux in the transformer core (delayed closing strategy). The residual magnetic flux of each transformer phase resulting from de-energization is calculated using the mathematical integral of the transformer voltage. When energizing the power transformer, the closing angle of the circuit breaker is adjusted in such a way that the prospective magnetic flux produced by the energization matches or equals the residual magnetic flux in that phase. The two other phases are closed n half cycles after the zero crossing voltage edge preceding the first phase to be closed.
While these two documents disclose the effectiveness of method to mitigate inrush current using independently-operated pole circuit breaker, there is a need to find a similar approach for simultaneous-operated poles breaker. For example, many transformers installed on medium-voltage systems are fed using a three-phase CB having simultaneous pole operation.
U.S. Pat. No. 8,310,106 discloses a method to mitigate inrush current for power transformer being energized through a simultaneous closing of three-phase circuit breaker or a non-phase segregated operation-type circuit breaker, without providing the circuit breaker with a resistor or other equipment. Such method of U.S. Pat. No. 8,310,106 defines an area of interest where all the three phases must be energized simultaneously to mitigate the inrush current. This area of interest is defined as the region in which the polarities of the steady-state magnetic flux and the residual magnetic flux coincide for all three phases. Also, this said method defines that one of the points to be targeted inside this area of interest is the intersection of the steady-state magnetic flux and the residual magnetic flux for the phase with the smallest residual magnetic flux among the phases of the three-phase transformer. Also, this same method defines another point on which the simultaneous closing of the three phases should occur. This point is the peak value of the steady-state magnetic flux of the phase that is having the largest residual magnetic flux. This method has limitations since there exist cases where the polarities of the steady-state magnetic flux and the residual magnetic flux can't coincide in the same region for all three phases. For example, in case no delta connection is present on power transformer, the sum of the three residual magnetic fluxes is not 0 all the time and they are all having the same polarity. Also, nothing can ensure that the intersection of the steady-state magnetic flux and the residual magnetic flux for the phase with the smallest residual magnetic flux among the phases of the three-phase is always located inside the area of interest described on the claim 1 of the U.S. Pat. No. 8,310,106.
There is thus a need for a new technique to potentially mitigate the shortcomings of the prior art and aiming at reducing the transients such as, but not limited to, inrush currents and voltage transients resulting from a transformer energization event using a three-phase circuit breaker using simultaneous pole operation or independent pole operation (i.e. having all three phases operated at the same time for a certain matter), regardless to the magnetic flux pattern inside the said transformer. Thus, there is a need to support all the possible transformer configurations such as, but not limited to, Y-Δ, Y-Y, Δ-Δ and Δ-Y with either floating or grounded neutral, and any vector groups.